1. Field of the Invention
This invention relates to a method for fabricating a compound semiconductor epitaxial wafer and a vapor phase growth apparatus using the method. More particularly, the present invention concerns a method for fabricating a compound semiconductor epitaxial wafer which is composed of elements belonging to a periodic table groups III and V and suitably used for fabrication of a light emitting diode, and a vapor phase growth apparatus using the method.
2. Description of the Related Art
In order to manufacture a red-, orange- or yellow-color light emitting diode, there is used a compound semiconductor epitaxial wafer in which an epitaxial layer of gallium arsenide phosphide GaAs.sub.1-a P.sub.a (where a is a real number satisfying a relationship of 0.ltoreq.a.ltoreq.1) having constant alloy compositions (1-a) and a of gallium arsenide GaAs and gallium phosphide GaP respectively is formed on a single-crystalline substrate of gallium phosphide GaP or gallium arsenide GaAs. The emitted light wavelength of the light emitting diode is determined by the alloy composition a, which is 0.9 for the yellow-color light emitting diode, 0.65 for the orange-color diode and 0.57 for the red-color diode.
A compound semiconductor epitaxial wafer 1 having such an epitaxial layer of the gallium arsenide phosphide GaAs.sub.1-a P.sub.a as mentioned above is, as shown in FIG. 3, made up of a single-crystalline substrate 2 of, e.g., n type gallium phosphide GaP, an n type gallium phosphide GaP epitaxial layer 3, an alloy composition gradient layer 4 of n type gallium arsenide phosphide GaAs.sub.1-x P.sub.x (0.ltoreq.x.ltoreq.1) having an alloy composition (1-x) of gallium arsenide GaAs varies in the growth direction of the epitaxial layer, an alloy composition constant layer 5 of gallium arsenide phosphide GaAs.sub.1-a P.sub.a (0.ltoreq.a.ltoreq.1) having a constant alloy composition (1-a) of gallium arsenide GaAs, and an alloy composition constant layer 6 of n type gallium arsenide phosphide GaAs.sub.1-a P.sub.a having a constant alloy composition (1-a) of gallium arsenide GaAs and is doped with nitrogens N as isoelectronic traps, which are sequentially formed on the single-crystalline substrate 2.
The term "compound semiconductor epitaxial wafer" as used in the present specification refers to a wafer having an epitaxial layer of compound semiconductor. Further, the term "compound semiconductor wafer" as used herein refers to a single-crystalline substrate of compound semiconductor or a compound semiconductor epitaxial wafer. Furthermore, these gallium arsenide phosphide GaAs.sub.1-x P.sub.x alloy composition gradient layer 4, gallium arsenide phosphide GaAs.sub.1-a P.sub.a alloy composition constant layer 5, and gallium arsenide phosphide GaAs.sub.1-a P.sub.a alloy composition constant layer 6 doped with nitrogen N will sometimes be generally referred to as the gallium arsenide phosphide GaAsP layers.
In order to grow any of the epitaxial layers 3, 4, 5 and 6 on the compound semiconductor wafer to fabricate the aforementioned compound semiconductor epitaxial wafer 1, there has conventionally been used such a vapor phase growth apparatus 20 as shown, e.g., in FIG. 4.
In the vapor phase growth apparatus 20, compound semiconductor wafers 21a, 21b and 21c are placed on a wafer holder 27 disposed inside a reaction furnace 29 so that the reaction furnace 29 is heated by a heater (not shown) located outside the reaction furnace 29 while a hydrogen H.sub.2 gas as a carrier gas 22 is introduced into the reaction furnace 29.
Collectively supplied together with the carrier gas 22 from a gas inlet 24 at one end of the reaction furnace 29 to a gas outlet 26 at the other end thereof are a group III source gas 23 containing gallium chloride GaCl, and a group V source gas 25 containing phosphine PH.sub.3 and/or arsine AsH.sub.3. The group III source gas 23 and group V source gas 25 react with each other on the compound semiconductor wafers 21a, 21b and 21c to grow epitaxial layers.
The above vapor phase epitaxial growth method, however, has a defect that, since the group III source gas 23 and group V source gas 25 are collectively supplied from one end of the reaction furnace 29, the epitaxial layer formed as grown on the wafer placed closer to the upstream side is thicker and the epitaxial layer on the wafer placed closer to the downstream side is thinner, because the downstream side has less source gases. This is because most of the group III source gas 23 and group V source gas 25 react on the side closer to the gas inlet 24 with the result that a relatively large amount of reaction product deposits on the upstream-side wafer; whereas, the residual source gases react on the side closer to the gas outlet 26 with the result that a relatively small amount of reaction product deposits on the downstream-side wafer. The thickness of the epitaxial layer greatly varies between the upstream and downstream wafers, and the maximum of the variation sometimes reaches 3 or 4 times the minimum of the variation in the reaction furnace.
Since the thickness of the epitaxial layer is associated with characteristics of emitted light wavelength, luminance, forward voltage, etc., variations in the thickness of the epitaxial layer will cause variations in the above quality characteristics.